Since the start of the development of electronic components and integrated circuits, the semiconductor industry has been continuously concerned with the task of increasing the integration density and for this purpose, inter alia, reducing the size of memory cells to an ever greater extent. These efforts to miniaturize electronic components, such as memory cells, are increasingly reaching the physical limits of the materials used, and the principle of information storage.
In the case of dynamic RAM (DRAM), the volatility of the information storage represents a major problem. The charge which is stored in the capacitor and thus the stored information must be refreshed periodically, and this is normally done at intervals of a few milliseconds. In the case of static RAM (SRAM), in contrast, no signals need be produced for refreshing (apart from the operating voltage) in order to maintain the data. SRAM memories can also be integrated in a chip. One disadvantage of SRAM memories in comparison to DRAM is that they require a large area. A further disadvantage is that SRAM store data only for as long as the operating voltage is applied. Like DRAM, SRAM is thus a volatile memory medium.
This volatility problem has been approached by the development of various technologies, such as the development of FRAM, MRAM and, in particular, flash memory cells, in which a charged floating gate is used for information storage. FRAM, MRAM and flash memory cells represent RAM types which are non-volatile.
DRAM and flash memory cells are subject to the further problem that, as the miniaturization of the memory cells progresses, the amount of charge which can be kept in a cell also becomes ever smaller (in this context, see, inter alia “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandary?”, M. N. Kozicki, M. Mitkova, J. Zhu, M. Park, C. Gopalan, Proc. VLSI (2002)). However, the reliability of information storage decreases as the amount of charge falls. Memory cells which are based on capacitive charging no longer operate satisfactorily in this “low-energy” range owing to the lower voltages and current levels resulting from the miniaturization.
These and other problems have led to the development of new memory technologies in recent years, which are based on concepts other than capacitive charging. In this case, the non-volatile CBRAM memory cells, which have been mentioned further above and on which the memory cells according to the present invention are based, have been found to be highly promising.
CBRAM memory cells comprise two electrodes and a thin layer composed of a solid electrolyte, which is arranged such that it makes contact between the two electrodes. The solid electrolyte contains a metal. The solid electrolyte and the metal together form a solid solution. When a sufficiently high electrical voltage is applied to the memory cell, the dissolved metal forms cations which migrate to the cathode through the solid electrolyte under the influence of the electrical field, in order to be deposited there as metal (M. N. Kozicki, M. Ynn, L. Hilt, A. Singh, Electrochemical Society Proc., volume 99-13, (1999), 298; M. N. Kozicki, M. Yun, S. J. Yang, J. P. Aberouette, J. P. Bird, Superlattices and Microstructures, volume 27, No. 5/6 (2000), 485-488; and R. Neale, “Micron to look again at non-volatile amorphous memory”, Electronic Engineering Design (2002)).
Solid electrolytes may be crystalline or amorphous solids. The solid electrolytes which are used in CBRAM cells are normally composed of amorphous solids, which can also be referred to as amorphous matrices or glasses (G. Saffarini, Phys. Stat. Sol (A), 170, 23 (1998)). The solid electrolytes are very particularly preferably chalcogenide compounds, in particular chalcogenide glasses (see M. N. Kozicki, loc. cit., R. Neale, loc. cit.).
Chalcogenides are compounds in which one or more elements from the sixth main group in the periodic table (oxygen, sulfur, selenium, tellurium) form the more electronegative components, which are referred to as oxides, sulfides, selenides and tellurides. Sulfides and selenides have a pronounced tendency to form amorphous solids. They are thus used with particular preference for the production of CBRAM cells. Oxides can likewise be produced as amorphous layers, but their structure (microstructure) is in general so dense that the ionic mobility of the metallic component is too low. Sulfides and, in particular, selenides have a more open microstructure and are thus preferable from the aspect of switching speed in CBRAM cells. One problem, however, is that the non-oxidic chalcogenide compounds have insufficient thermal stability with respect to the crystallization.
Germanium from the fourth main group in the periodic table can be used as a more electropositive chemical element in the chalcogenide compounds. These so-called IV-VI compounds are suitable for use as amorphous matrices for CBRAM memory cells, with germanium sulfide and germanium selenide representing particularly preferred IV-VI compounds for use as amorphous matrices in CBRAM memory cells. Alternatively, however, silicon selenide or silicon sulfide are also suitable for use as amorphous matrices for CBRAM memory cells.
The selenium content in germanium selenide GexSe1-x and the sulfur content in germanium sulfide GexS1-x can be varied over a wide range. Particularly highly suitable glasses with ideal characteristics are obtained when x is in the range from 0.1 to 0.5 and is, for example, 0.33. In an entirely general form, x must have a value such that the corresponding selenide or sulfide can easily form a stable glass, which is able to conduct solid ions. All selenide and sulfide glasses are referred to in the following text as Ge—S and Ge—Se chalcogenide compounds, irrespective of the respective value of x.
Under the influence of an electrical field, the metal which is contained in the amorphous matrix which forms the active material of the CBRAM memory cell, in particular in the chalcogenide glass, forms cations which migrate through the amorphous matrix to the cathode under the influence of this field. One particularly highly suitable metal is silver (Ag). Silver is highly electrically conductive in the metallic state. It can easily be ionized and, in the ionized state (as Ag+) has the required mobility in the amorphous matrix of the chalcogenide glass, so that it easily migrates to the cathode, in order to be reduced to metal there. The amount of silver which can be absorbed by Ge—Se and Ge—S compounds depends on the quantity ratio Ge/S or Ge/Se. This saturation concentration is normally a few tens of percent by atomic weight and is, by way of example, 47.3 percent by atomic weight of silver for Ge0.2Se0.8 (M. N. Kozicki, M. Mitkova, J. Zhu, M. Park, C. Gopalan, loc. cit.).
The cathode is typically composed of an inert metal, such as aluminum or tungsten. The cathode can also be composed of tantalum, titanium, refractory materials, such as conductive oxides, nitrides, of nickel or of heavily n-doped or heavily p-doped silicon, or alloys of the stated materials.
The anode is a metal or a chemical compound whose cations are able to migrate as ions through the solid electrolyte. The anode is preferably composed of a metal which is also contained in the solid electrolyte. In consequence, the amount of metal which is deposited electrochemically on the cathode can be replenished by oxidation of the anode. It is thus particularly preferable to use an anode composed of silver or of silver compounds, such as Ag2S, Ag2Se, Ag—Al—S, Ag—Al—Se.
Solid electrolytes, such as the preferred Ge—Se—Ag glasses and Ge—S—Ag glasses which conduct ions over a wide temperature range, can be produced by photolytic dissolving of silver in a thin layer of the solid electrolyte. (Kozicki, M. N.; Mitkova M., Zhu, J.; Gopalan, C. “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandary?”, loc. cit.).
Before an electrical voltage is applied for the first time, the resistance between the anode and the cathode of the CBRAM memory cell is high. The high resistance value is associated with the off state or the logic 0, and can be read by suitable readers. Application of a small voltage, of typically a few 100 mV, between the cathode and the anode of the memory cell switches the CBRAM memory cell to a state which is characterized by a considerably lower value of the electrical resistance, with this value in some cases being several powers of ten lower. The metal of the anode is oxidized during a switching process. The cations that are formed migrate through the solid electrolyte to the cathode area, where they are reduced to metal again. A metallic or metal-rich phase is electrochemically deposited in or on the solid electrolyte. In consequence, the cathode is electrically conductively connected to the anode. This process, which is referred to as “Conductive Bridging”, gives CBRAM memory cells their name.
This state of the memory cell, which is characterized by the considerably lower electrical resistance, is associated with the on state or the logic 1. Like the off state (the logic 0), it can be read by determining the cell resistance.
CBRAM memory cells are non-volatile memory media. The low-resistance (highly conductive) state and the high-resistance state of the memory cell are both maintained even without any power being supplied, at room temperature.
Storage in CBRAM cells is reversible. The metal that has been deposited on the inert cathode (now the anode) is dissolved again with oxidation by application of a voltage of similar magnitude but with an opposite mathematical sign, that is to say the cathode becomes the anode, and the anode becomes the cathode, and migrates in the form of cations back through the solid ion conductor to the reactive anode (now the cathode). The conductive bridge is thus interrupted, so that the CBRAM cell reverts to the high-resistance state. This deletion process takes place just as quickly as the writing process (switching to the low-resistance state). These write/delete cycles can be repeated virtually indefinitely.
CBRAM cells represent an alternative of major interest to the memory cells that have been known in the past and can be used to overcome the problems outlined further above. However, the electrical characteristics and the thermal stability have not been completely satisfactory in the past.
A first problem is that the memory cells have an excessively high electrical resistance in the low-resistance, that is to say conductive, state (=on state) and very particularly in the high-resistance, that is to say considerably less conductive, state (=off state). This high resistance is disadvantageous from the design point of view for reading the CBRAM memory cells.
A second problem is that the electrical resistance in CBRAM cells rises with time and this affects, in particular, the electrically conductive on state. This reduction in the electrical conductivity leads to it becoming ever more difficult to read information that is stored in the cells (so-called “retention loss”). This can lead to the reading process lasting for an ever longer time as a result of the matching of the electrical resistance in the on state and in the off state. In the extreme, the stored information becomes unreadable.
A third problem is that the memory cells which have been investigated in the past do not have sufficient thermal stability for standard integration in a BEOL-CMOS process. The BEOL-CMOS process comprises method steps such as formation of wiring layers, contact layers (local interconnects), isolation layers and passivations. In this case, oven processes are carried out in addition to methods for deposition of layers, polishing processes and etching processes. In the oven processes, the electronic components are heated to temperatures which may typically be up to 400 to 450° C. These temperatures are too high for solid electrolytes formed from an amorphous or partially amorphous material, such as those used in CBRAM memory cells, in particular for chalcogenide glasses. The cells that are known at the moment are thus damaged at these temperatures. This is because the crystallization of the storage medium which is composed of an amorphous or partially amorphous solid electrolyte, in particular of a chalcogenide glass, starts even at considerably lower temperatures. The migration of ions in the storage medium thus becomes more difficult or impossible, resulting in failure of the memory cell.
Various investigations have been carried out into the thermal stability of chalcogenide glasses. Investigations by Saffarini have shown that the thermal stability of Ge—S—Ag glasses decreases as the silver content increases, and the tendency for conversion of the glass to a crystalline material increases (G. Saffarini, Phys. Stat. Sol. (A), 170, 23 (1998) “Experimental Studies on Ge—S—Ag Glasses”). One simple measure of the thermal stability of glasses is the difference (TC−Tg), where TC means the crystallization temperature and Tg the glass transition temperature. The greater this difference, the more stable is the glass state. Saffarini carried out investigations with Ge—S—Ag glasses whose silver content was varied in 5% steps from 5 to 30 percent by atomic weight. During the process, it was found that the thermal stability of Ge40Ag5S55 (TC−Tg=133 K) decreases continuously to Ge20Ag30S50 (TC−Tg=103 K).
Ramesh et al. have carried out crystallization studies in germanium telluride glasses containing copper with the formula CuxGe15Te85-x, with x being varied in the range from 2 to 10 percent by atomic weight (Ramesh K. et al., J. Phys. Cond. Matter 8, (1996) 2755). The thermally most stable glass has the composition Cu5Ge15Te80 (TC−Tg=98 K). If the copper content is increased further (6, 8, 10 percent by atomic weight), the thermal stability of the glass decreases ever further (TC−Tg=90, 73, 43 K).
A further problem with the memory cells that have been known so far is that they do not have adequate data retention at high temperatures. The electrical conductivity falls or is lost completely over time, which makes the reading of the cell more difficult, makes it take longer, or makes it impossible.
For these and other reasons there is a need for the present invention.